Generic extension of marks with extension ids becomes possible through a...

... d3'like "call" extension point method.

The function receives the corresponding mark as first argument (and `this` is also the mark).
One scenario that is particulary usefull and that is currently not possible through the "add" extension point is to anchor new marks to the extension id mark.

Example use:
````javascript
extensionPoints: {
    bar_call: function() {
        return this.anchor('right').add(pv.Label)
            .text("Hello");
    }
}
````

@pamval please review.